Endoscopes and the aforementioned viewing tubes, in addition to applications in medicine, are also used in technology for observations in hollow spaces such as bodily cavities, combustion chambers of engines, power systems, and the like. Depending on the depth of the cavity to be investigated, various transmission lengths need to be bridged by means of the rod lens system.
Therefore, image transmission systems called relay lens systems are also used, especially in rigid endoscopes as well as other viewing tubes such as technoscopes, periscopes, and the like. Endoscopes consist of an eyepiece portion, containing the eyepiece, as well as an oblong shaft in which additional optical components are contained. These additional optical components consist of the objective found at the front (distal) end of the endoscope and the image transmission or image relay lens system. The objective receives the scene to be observed and in an initial intermediate plane projects an intermediate image. This intermediate image is again configured by an image transmission or relay lens system and conveyed to the next intermediate image plane, from where an additional image transmission or relay lens system can assume the conveying task to the next intermediate image plane, until the image reaches the eyepiece.
The use of rod lenses to construct image transmission systems was proposed for the first time by Professor Hopkins in GB-PS 954 629. According to this fundamental proposal from Professor Hopkins, every image transmission system consists of two biconvex rod-shaped lenses and at least two additional meniscus lenses. Through the combination of rod-shaped biconvex lenses with meniscus lenses, it is possible, first, to achieve strong image brightness and, second, to compensate well for image errors such as astigmatisms and image field curvature, which can be a hindrance in image transmission systems.
As a result of the works of Professor Hopkins, a number of modified image transmission systems, which also used rod lenses, were developed and described.
An image transmission system of this kind is known, for instance, from EP 0 628 843. This known improved image transmission system consists in the embodiment in FIG. 6 of an internal and two outer rod lenses, where one convex lens and an additional concave lens element are cemented onto the outer rod lenses, on both sides in each case. The resulting lens structure is here convex-concave. The center lens is biconvex and of a single piece.
An additional image transmission system is seen in U.S. Pat. No. 5,805,345, in which a rod lens system is mentioned as state-of-the-art. In this system the center lens is configured either as a biconcave lens with biconvex lenses cemented on both sides or as a biconvex lens with meniscus elements cemented on both sides. In both cases the external rod lenses are identical and biconvex.
U.S. Pat. No. 4,168,882 presents a rod lens system with two rod lenses, affixed directly around the center plane, which are concave on the side turned inward and convex on the side turned outward, and two convex rod lenses affixed to them on the outside. The inner or outer rod lenses can be connecting lenses, and on the surfaces which the inner and outer rod lenses are turned toward, one meniscus element is cemented onto both the inner and the outer rod lenses. The connecting rod lens resulting from this is convex on the meniscus side, and the external rod lenses are in addition symmetrical to one another with respect to a symmetry plane of the image transmission system located perpendicular to the optical axis between the inner rod lenses.
In addition, in U.S. Pat. No. 5,805,345 a rod lens system consisting of two identical biconvex outer rod lenses and one smaller center connecting lens is described as the basic state-of-the-art for U.S. Pat. No. 5,805,345. In this case the connecting lens main element of the aforementioned center connecting lens is biconcave in one embodiment and has biconvex lens elements cemented to it, while according to an additional embodiment it is biconvex and has meniscus elements cemented to it.
In the construction of these embodiments, the outer rod lenses are set at a distance from the center connecting lens, possibly by means of distancing tubes, and, as previously mentioned, the center connecting lens is substantively shorter than both outer rod lenses.
These image transmission systems make possible a good correction of the astigmatism and image field curvature, but have the disadvantage that the transmitted proximal image is markedly darker than the distal starting image.